The present invention relates to an image forming apparatus and method.
In general page printers, a latent image formed on a photosensitive drum and developed thereon is directly transferred onto a printing medium. On the other hand, in page printers using an intermediate transfer medium, an image is once transferred onto an intermediate transfer medium such as a belt, and is then transferred onto a conveyed printing medium. This mechanism is advantageous in that it is not necessary to provide a plurality of process units in a color printing apparatus.
In general, in color printing, a plurality of colors, such as CMYK, are used.
In the case of a plurality of colors, to print them onto a printing medium using one path is the simplest control method.
However, this method requires process units corresponding to the respective colors, resulting in an increase in the size of the apparatus and in cost.
On the other hand, in intermediate-transfer-type printing apparatuses, when color printing is executed, each of the colors is transferred onto the intermediate transfer medium whenever the intermediate transfer medium rotates through one rotation, and all the colors are simultaneously transferred therefrom onto a printing medium. This method does not require a process unit corresponding to each color, and hence is widely employed in, in particular, a small-sized intermediate/low-speed machine.
If the intermediate-transfer-type printing apparatus is compatible with A3-size, the circumference of the intermediate transfer medium is equal to or greater than the length of A3-size sheets. When A4-size printing is executed using this printing apparatus, a widely known method is employed, in which image data corresponding to two pages is formed on the intermediate transfer medium and is then simultaneously transferred onto a printing medium such as paper.
This method is advantageous in enhancing the throughput since two sheets of the printing medium are always continuously conveyed as if they are coupled.
In the meantime, when double-sided printing is executed using, for example, a stackless ADU (Auto Duplex Unit), paper sheets must be conveyed with an interval that enables each sheet to be reversed, since each sheet is reversed after printing is executed on one side. Also in the case of executing a post-process such as offset paper discharge, stapling, etc., using a finisher, a paper interval that enables the post-process is needed. Thus, appropriate paper intervals are necessary for these processes.
On the other hand, in the above-described conventional intermediate-transfer-type printing apparatus, when printing is executed using the intermediate transfer medium, image forming is executed in accordance with the rotation of the intermediate transfer medium, therefore conveyance of the printing medium must also be adjusted to the rotation.
For example, when image forming of two pages is executed, the interval between printing medium sheets of pages 1 and 2 is identical to that of images formed on the intermediate transfer medium.
Thus, in the prior art, when the above-mentioned process is executed, a sheet interval more than required is secured, at the sacrifice of the throughput, by printing only a one-page image on the intermediate transfer medium even if the image is an A4-size image, or by temporarily stopping the intermediate transfer medium or making it run idle.